1. Field of the Invention
The present invention relates to a lithium secondary battery comprising a wound electrode assembly having a negative electrode comprising a negative electrode active material alloying with lithium, a positive electrode comprising a positive electrode active material containing a transition metal composite oxide, a separator disposed between the positive and negative electrodes, wherein the positive electrode, the negative electrode, and the separator are spirally wound, the wound electrode assembly having a cylindrical hollow space at the winding axis and in the vicinity thereof.
2. Description of Related Art
Rapid advancements in size and weight reductions of mobile information terminal devices such as mobile telephones, notebook computers, and PDAs in recent years have created demands for higher capacity batteries as driving power sources for the devices. Because of their high energy density and high capacity, lithium secondary batteries are widely utilized as the driving power sources for such mobile information terminals.
A lithium secondary battery generally employs a positive electrode containing a positive electrode active material made of a lithium-transition metal composite oxide, a negative electrode containing a negative electrode active material made of a carbon material capable of intercalating and deintercalating lithium, such as graphite (theoretical capacity: 372 mAh/g), and a non-aqueous electrolyte in which an electrolyte made of a lithium salt, such as LiBF4 and LiPF6, is dissolved in an organic solvent such as ethylene carbonate and diethyl carbonate. In this kind of battery, the charge-discharge operations are performed by migration of lithium ions between the positive and negative electrodes.
As the number of functions of mobile information terminals has increased, the power consumption of the devices has been increasing. Accordingly, demand has been increasing for lithium secondary batteries with further higher energy. An effective means to meet such demands is to use a material having a greater energy density as a positive/negative electrode active material. Taking this into consideration, it may be conceivable to use metallic lithium, which has a greater theoretical capacity than graphite, as a negative electrode active material. Although use of metallic lithium as a negative electrode active material of a lithium secondary battery enables a high charge-discharge capacity, there has been a problem that the metallic lithium causes dendrite formation on the negative electrode, which can lead to battery internal short circuiting.
In view of the problem, it has been proposed to use metals such as Si, Sn, and Al that are capable of alloying with lithium as a negative electrode active material that is expected to achieve a high charge-discharge capacity without causing the foregoing problem. However, when such a metal capable of alloying with lithium is used as the negative electrode active material, the volume of the active material changes greatly due to repeated charge-discharge cycles. Consequently, the negative electrode active material pulverizes and peels off from the negative electrode current collector, resulting in the problem of insufficient current collection performance.
In view of such a problem, Japanese Published Unexamined Patent Application No. 2004-241329 proposes a method for obtaining a sufficient current collection performance even when a material that causes a great volumetric change is used as the negative electrode active material, by combining the negative electrode active material with the negative electrode current collector to prevent the peeling-off of the negative electrode active material from the negative electrode current collector. The method includes, for example, applying a slurry containing a Sn alloy, a binder agent, a diluting solvent onto the surface of the negative electrode current collector, then drying, and thereafter electroplating a metal with a low capability of chemically combining with lithium. Nevertheless, even with such a method, the peeling-off of the active material cannot be prevented sufficiently, and sufficient cycle performance cannot be obtained.
Various other approaches have been proposed. In one example, a negative electrode active material made of an alloy with a high conductivity and a carbon conductive agent are uniformly dispersed to improve the conductivity of the negative electrode active material layer, so that the electrode structure can ensure sufficient current collection performance when the constituent pressure acting on the negative electrode within the battery is sufficient even if the negative electrode active material peels off from the negative electrode current collector because of the expansion of the negative electrode active material. An evaluation of such an electrode structure has been made on a cylindrical battery, which can obtain uniform constituent pressure easily. However, since the outer diameter of the wound electrode assembly is invariable in the cylindrical battery, stress acts toward the inside of the wound electrode assembly, and consequently, deformation 21 (swelling) occurs as illustrated in FIG. 6 such that a portion of the wound electrode assembly 5 sticks out into a hollow space 14 formed in the center area of the winding axis. This causes breakage of the electrode plates, resulting in abrupt capacity degradation. This problem arises also when the negative electrode active material used is a graphite-based one, which causes less expansion. However, in this case, the problem is less serious since it does not occur until the number of charge-discharge cycles is built up. On the other hand, when the negative electrode active material used is an alloy-based one, which causes greater expansion, the problem is far more serious because the deformation starts from the initial stage of cycles and the deterioration of the charge-discharge capacity tends to be accelerated.
In view of such circumstances, there has been a proposal to provide a center pin in the hollow space of the wound electrode assembly to prevent breakage of the electrode plate resulting from the deformation of the wound electrode assembly and to thereby prevent internal short circuits (see Japanese Published Unexamined Patent Application No. 2006-134760).
Nevertheless, when a negative electrode active material that undergoes large expansion is used, deterioration of the cycle performance resulting from the battery internal short circuits or the like cannot be prevented by the just-described configuration alone. When it is attempted to prevent the breakage of the electrode by suppressing the deformation of the wound electrode assembly inward, the force cannot escape toward the inside of the battery, but instead acts in such a direction as to expand the diameter and in such a direction as to squash the separator disposed between the positive and negative electrodes. In this case, especially when a separator made of a soft and flexible material is used, the thickness thereof reduces and squeeze-out of the electrolyte solution occurs, deteriorating cycle performance. In addition, breakage of the separator may occur, which can result in internal short circuits.
The present invention has been accomplished in view of such problems as described above, and it is an object of the invention to provide a lithium secondary battery that achieves a higher battery capacity by employing a negative electrode active material that expands largely and at the same time dramatically improve various characteristics of the battery, such as cycle performance.